In the process of manufacturing silicon microchips, light is directed through a reticle mask to etch circuits into a silicon wafer disk. The presence of dirt, dust, smudges or other foreign matter on the surfaces of the reticle mask or the silicon water is highly undesirable and adversely affects the resulting circuits. As a result, the reticles and silicon wafers are necessarily inspected before use. One common inspection technique is for a human inspector to visually examine each surface under intense light and magnification. However, debris that is smaller than can be visually detected impairs the resulting microchips.
Laser surface inspection devices have been developed for inspecting the surface of silicon wafers to accurately detect small particles. Examples of such devices are disclosed in Alford et al U.S. Pat. No. 4,376,583 issued Mar. 15, 1983, and Moran U.S. Pat. No. 4,630,276 issued Dec. 16, 1986. In these known laser surface inspection systems, a laser beam is traversed across the surface of the silicon wafer and the reflections from the wafer are collected and analyzed to provide information about any debris present on the wafer surface. In the absence of debris, all of the light is specularly reflected from the surface. In locations where the beam strikes surface debris, the light is scattered. By separately collecting the scattered and specularly reflected light, the inspection device can accurately determine the surface conditions of silicon wafer disks.
However, such devices are not suitable for inspecting a reticle. The reticle has an extremely thin coating on the surface to form the masking. The edges of the coating tend to scatter light which causes spurious detections of foreign matter. The complexity of the circuits would cause the inspection device to determine that an otherwise clean reticle is saturated with debris. The use of such prior art devices is further unsuitable because only one side of the workpiece is inspected. Both sides of a reticle must be clean and therefore both sides need to be inspected.